The need to detect motor vehicles for traffic signal control, parking, and access control applications has existed for a substantial period of time. Inductive loop vehicle detection systems are used to provide specific vehicle location in the roadway for signal timing, vehicle speed determination, and vehicle classification. In addition, inductive loop vehicle detectors are used extensively in entry control applications such as electric gates or doors in buildings, garages, residential applications, parking lots, and other controlled access areas.
Typically, inductive loop vehicle detectors have in common an oscillator device, which is contained in the vehicle detector system and is connected to the remote roadway loop system utilizing an isolation transformer and a transmission cable assembly. The oscillator contained in the vehicle detector system will operate at a resonant frequency determined by the value of the fixed capacitors located in the oscillator circuit and the equivalent inductance of the remote roadway loop. In the applications above, the inductance of the loop system decreases and the resonant frequency of the loop system increases from a reference value when a vehicle enters the loop perimeter, or is in close proximity to the roadway loop. The frequency shift of the oscillator system due to a normal sized passenger vehicle entering the loop area is generally only 1% or 2% of the inductance value of the system without a vehicle being present. A small motor vehicle, such as a small motorcycle, may only change the frequency 0.05%.
The presence or absence of a motor vehicle is determined by the vehicle detector system measuring the inductance of the roadway loop and comparing this value with a known inductance value which represents the inductance of the loop with no vehicle present. If the inductance value is presumed to be of sufficiently lower than the reference value, the vehicle detection system outputs a logic signal to external devices such as traffic controllers or gate operator systems. As long as the inductance value remains sufficiently low, the vehicle detection system will continue to output the same signal (referred to commonly as the “detect” signal).
Inductive loop vehicle detection systems are both emitters and receptors of electromagnetic fields. These electric fields are known to be of very low power. However, if the roadway loops are in close proximity to each other, the electromagnetic field from one roadway system inductively couples into other loop systems. The result of this loop field coupling by multiple vehicle detectors systems is an interference to other individual detector oscillator systems. The effect of two or more vehicle detector loops coupling inductively with each other is commonly referred to as crosstalk. The result of this electromagnetic field coupling is that each system tries to change the frequency of the other system. This will result in one or both systems operating at either a higher or lower frequency than it would without the influence of the other system.
Mutual interference between vehicle detectors has existed for a substantial period of time and can be severe, particularly if a loop system is operating with a resonant frequency close to another system's resonant frequency. The interfering signal will be a modulation product consisting of all frequencies of the various detectors plus the sum and difference of all of the detector loop frequencies. The operation of vehicle detection systems, each with a slightly different frequency, is not unlike that of a plurality of radio transmitters operating on the same frequency. This situation is commonly referred to as “transmitter jamming.”
Crosstalk in vehicle detection systems can cause random false vehicle detect signals from one or more detector systems. It is also common, if the detector system is experiencing crosstalk, to observe a vehicle detector that will not output a detect signal when a vehicle is present over the roadway loop. This is also an undesirable situation that can result in disruptive equipment operation and will cause traffic lights and/or gate systems to malfunction.
The interference between various detector systems within a given area has been dealt in various ways. For example, the individual systems have included manual systems for varying the operating frequencies of the loop systems. This has been accomplished in the past, and is still being accomplished, by manually adding (or subtracting) capacitors or inductors of different values that cause the natural resonance frequency of the roadway loop to shift to a value different than any other systems in the vicinity. The selection of the various frequencies must be coordinated among all of the detector systems that are suspected to have roadway loops that are in close enough proximity to each other to suffer from interference. The manual selection of different frequencies is generally accomplished at the time of installation of the devices and it does not take into account the change of the values of all the components in the resonant circuit with both time, temperature, and other variables. Many times a frequency selection is made only to have the problem of crosstalk reappear at a future time as changes in the value of the oscillator components and the roadway loops occur.
Another technique for dealing with interference has been the use of sequential scanning of more that one detector system. The detector systems are controlled by a master sequencing device, which only operates one oscillator at a time in a controlled sequence. Systems have been in existence for a number of years that use this sequential scanning principal. A drawback to sequential scanning is that fact that the operation of multiple detection systems must be synchronized with each other. Another drawback is that only the loops that are controlled by this single device are corrected and typically these types of systems can only manage up to four detection loops simultaneously. These multiple detection devices have no communication with other nearby similar devices and therefore only the scanned channels of detection common to this single device are exempt from interference from each other. In a typical traffic intersection application, the total number of roadway loop systems may be a large number and the synchronization of only groups of four, is of limited value in solving the overall crosstalk problem.
While some techniques for dealing with interference between vehicle detectors exist, there is still a need for techniques that are easy to implement and that are applicable to multiple detection systems.